utils: atomic_vector: introduce future-returning `for_each` function
utils: atomic_vector: introduce future-returning `for_each` function

Introduce a variant of `for_each` function not requiring
`seastar::async` context.

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla